Add tests for dfs source=None Addresses #418
Add tests for dfs source=None
Addresses #418

michaelpacer/networkx,jcurbelo/networkx,chrisnatali/networkx,wasade/networkx,nathania/networkx,jakevdp/networkx,bzero/networkx,tmilicic/networkx,kernc/networkx,nathania/networkx,JamesClough/networkx,kai5263499/networkx,NvanAdrichem/networkx,farhaanbukhsh/networkx,blublud/networkx,cmtm/networkx,RMKD/networkx,SanketDG/networkx,andnovar/networkx,RMKD/networkx,blublud/networkx,dmoliveira/networkx,RMKD/networkx,jakevdp/networkx,kernc/networkx,debsankha/networkx,jni/networkx,ionanrozenfeld/networkx,dhimmel/networkx,dhimmel/networkx,sharifulgeo/networkx,farhaanbukhsh/networkx,kernc/networkx,dmoliveira/networkx,jakevdp/networkx,jni/networkx,jtorrents/networkx,nathania/networkx,OrkoHunter/networkx,blublud/networkx,aureooms/networkx,ghdk/networkx,ionanrozenfeld/networkx,harlowja/networkx,ltiao/networkx,jfinkels/networkx,dhimmel/networkx,sharifulgeo/networkx,dmoliveira/networkx,bzero/networkx,chrisnatali/networkx,harlowja/networkx,debsankha/networkx,ghdk/networkx,kai5263499/networkx,farhaanbukhsh/networkx,bzero/networkx,beni55/networkx,Sixshaman/networkx,aureooms/networkx,ionanrozenfeld/networkx,ghdk/networkx,kai5263499/networkx,debsankha/networkx,chrisnatali/networkx,jtorrents/networkx,sharifulgeo/networkx,yashu-seth/networkx,harlowja/networkx,jni/networkx,goulu/networkx,aureooms/networkx